Certain clamping devices for axially movable rods, e.g. piston rods of working cylinders, or fixed rods, are known. A clamping device of this kind is described in DE 37 07 046 A1. Clamping devices of this kind are connected to a moving component or device relating to the rod which is to be clamped, and the clamping force is generated for example by means of a tensioning piston with an inner cone and a clamping sleeve with an outer cone. As the level of clamping force is directly dependent on the level of spring force, sets of plate springs are always used for high retaining or clamping forces in the prior art, which depending on the requirements consist of single- or multi-layer plate springs lined up in rows in alternate directions and are arranged coaxially.
The mode of operation under high forces is as follows. To release the clamping sleeve from the rod, the tensioning piston is pushed by hydraulic pressure counter to the spring force of the spring assembly, so that the clamping sleeve is no longer acted upon at its outer cone and the rod is thus released. To initiate the clamping action, i.e. to act on the outer cone of the clamping sleeve, the hydraulic pressure is removed, so that the spring force of the spring assembly can come into effect.
Problems arise with clamping devices of this kind particularly when one of the plate springs forming the spring assembly breaks, as this causes the spring unit as a whole to lose most of its bias. This is associated with a loss of the ability to transmit force to the tensioning piston and consequently a substantial reduction in the clamping force or retaining force.
On the other hand, bodies concerned with safety in the workplace, such as the professional and trade associations, demand that when any operational locking/securing mechanisms are used for presses and injection moulding machines, there must be no loss of the safety function even if a spring should break, i.e. the retaining force of the clamping device and hence the dynamic course should still be preserved. Particularly in the field of presses and injection molding machines referred to above, where the clamping and retaining mechanisms have to withstand very great forces which could endanger the lives of the operating staff, it is essential to comply with these professional requirements, which determine whether a clamping device will be authorized and released for use.
In the past, clamping devices for meeting these requirements were fitted with a clamping jaw arrangement which allows a hydraulically driven piston rod to be safely secured. A clamping jaw arrangement of this kind is known for example from published German application DE-OS 23 33 491. A disadvantage of the known clamping jaw arrangements is that in practice, when the clamping jaws are used, the forces applied frequently lead to jamming which can only be freed again with difficulty depending on the magnitude of the forces occurring and absorbed and sometimes lead to damage or even destruction of the apparatus, if the jamming is serious.